1. Field of the Invention
The present invention relates to integrated circuit (IC) amplifiers, and more specifically to IC power amplifiers for driving three-wire stereo headphones in small portable stereo systems.
2. Description of the Related Art
Small portable stereo systems generally include a digital audio source, a digital volume control, a digital-to-analog converter (DAC), and an integrated circuit (IC) power amplifier for driving analog output signals to stereo headphones. The vast majority of these stereo headphones have a three-wire connection consisting of a left (L) terminal, a right (R) terminal, and a common (C) terminal. The IC power amplifier typically includes a first amplifier that drives a left (L) channel signal connected to the left (L) terminal and a second amplifier that drives a right (R) channel signal connected to the right (R) terminal. The common (C) terminal of the headphone is generally connected to system ground (GND).
Because of the low voltage supplies used to power these portable stereo systems, the dynamic range and maximum power delivered by typical power amplifiers to the stereo headphones are limited. For example, with a typical IC power amplifier and a low voltage supply, such as a 3 volt supply, the left (L) and right (R) channel outputs can be driven from approximately 0.5 volts to 2.5 volts. This signal range limits the maximum sine wave that may be delivered to the headphone to about 0.707 volts rms and limits the maximum power into each channel to about (0.707 V rms)2/32 Ω=15 mW. In addition, with a typical IC power amplifier, the left (L) and right (R) terminals must be AC coupled to the IC power amplifier through large capacitors to provide a low frequency pole for the left (L) and right (R) headphone speakers. For example, assuming right (R) and left (L) headphone speakers having a typical value of about 32 Ω, the coupling capacitors must be about 100 μF to place the low frequency RC pole at about 50 Hz. The cost of two large capacitors, such as 100 μF capacitors, is a significant problem for these types of systems.
To eliminate the need for the AC coupling capacitors, a third amplifier may be added to the IC power amplifier to drive the common (C) terminal of the headphone at a fixed common mode (CM) voltage (VCM). This fixed common mode driver approach, however, requires that the third amplifier have approximately twice the current drive capability of the left (L) channel and right (R) channel amplifiers at peak current loads. This additional high power third amplifier significantly increases the power consumption of the IC power amplifier and does not increase the maximum power deliverable to the headphone. For example, in a typical system, there is a particular volume control setting that will cause the power amplifier to just barely clip the signal when given a full scale signal from the digital audio source. Because the program material being processed by the digital audio source typically does not contain any full scale signals, the volume control will often have settings that exceed this clipping limit. Assuming that the output of the right (R) and left (L) channel drive amplifiers can swing from VCM−1 volts to VCM+1 volts without clipping, the maximum voltage that the IC power amplifier in the fixed common mode approach can deliver to the headphones without clipping is about 2 Vpp (volts peak-to-peak).